Whether it be the aroma of our morning coffee or the scent of a lover, the sense of smell is a critical feature of our daily life. The long-term objective of our research is to understand how olfactory information is processed in the mammalian brain. To address this question, we study the properties of neuronal circuits and synapses in the olfactory bulb and olfactory cortex, which are the first sites in the brain where olfactory information is processed. Our unifying hypothesis is that understanding the synaptic mechanisms of olfactory circuits is critical for revealing how the brain encodes our sense of smell. The experiments proposed employ patch-clamp recording techniques to study the functional properties of olfactory circuits in vivo and in vitro. Specific Aim 1 proposes to characterize the fundamental mechanisms governing odor representations in the piriform cortex. We hypothesize that odor-evoked excitatory input to cortical pyramidal cells in vivo reflects both direct sensory input from the olfactory bulb and associational (recurrent) connections between cortical pyramidal cells. Specific Aim 2 proposes to investigate the role of local inhibitory circuits that shape activity in piriform cortex. We hypothesize that distinct types of dynamic feedback circuits govern recurrent inhibition and are differentially recruited by physiologically relevant patterns of pyramidal cell activity. Specific Aim 3 proposes to investigate the role of long-range feedback connections from piriform cortex to the olfactory bulb. We hypothesize that excitatory projections from pyramidal cells regulate the initial stages of odor coding in the olfactory bulb. These experiments will provide new insight into the synaptic mechanisms of neural circuits underlying olfaction in the brain.